Because the RBS field can be used as a part of the Project Online security model, the system does not allow you to make this field a multi-value field. Also, the RBS is meant to mimic the "org chart" for your organization, which means that each user has a single location in the org chart and not multiple locations. Out of curiosity, why do you ask? What are you trying to accomplish? Please let us know and we will try to help you.